BIG PHYSICS, BIG QUESTIONS –

Parasite turns wasp into zombie then drills through its head

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.

A master manipulator

Ryan Ridenbaugh and Miles Zhang

By Colin Barras

Species:Euderus set waspHabitat: evergreen oak trees of southeastern US

The crypt gall wasp (Bassettia pallida) is a master manipulator. It parasitises the sand live oak tree, encouraging it to form hollow galls – or “crypts” – in its woody stems. Young wasps develop inside the crypts through the second half of the year, chewing their way out to emerge as adults the following spring.

At least, most of them do. Some crypt gall wasps don’t make it: they begin to chew their way out several months earlier than expected, but stop while the exit is still only as large as their head. Then they die, with their head blocking the hole (see photo, below).

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Now it appears that the manipulator is itself manipulated into an early death.

Kelly Weinersmith at Rice University in Houston, Texas, and her colleagues examined about a dozen of the “head blocked” crypts. In all but one case they found the larvae or pupae of a smaller wasp – Euderusset – living as a parasite inside the larger B. pallida wasp, slowly eating it.

Two crypt holes blocked by the heads of Bassettia pallida wasps

Sean Liu

As the researchers studied the crypts in more detail they noticed something else. Sometimes the B. pallida head that was blocking the hole had an even smaller, E. set-sized hole chewed through it – and no sign of the E. set parasite (see photo, below).

This set them wondering: what if the E. set wasp needs its host’s strong jaws to chew an exit hole out of the crypt. What if the E. set wasp manipulates the B. pallida wasp, encouraging it to make a small hole in the woody tissue months earlier than it should and block it with its head to keep the crypt sealed and prevent predators getting in.

Then, E. set can get on with the business of eating its host. When it is ready to leave, E. set simply chews through its host’s head to reach the outside world.

To find out if they were right, Weinersmith’s team brought wasp-infected stems into the lab. They identified crypts with head-blocked holes, leaving some as they were and securing a thin strip of bark over others.

The Euderus set wasp drills a hole in its host’s head

Sean Liu

The following spring, when E. set wasps were ready to emerge, those in the crypts covered with bark were about three times more likely to die stuck inside the crypt. They could chew through the B. pallida wasp head, but they couldn’t get past the barrier imposed by the strip of bark.

“E. set either has fairly weak jaws, or lacks the energy to finish the job,” says Weinersmith.

She thinks E. set might qualify as a “hypermanipulator” – a species that manipulates a species that, in turn, manipulates another.

Manipulating mechanism

“At this time we really don’t know what mechanism E. set is using to manipulate B. pallida’s behaviour,” says Weinersmith. “It’s possible that the parasitoid is releasing some compound – like a hormone – or multiple compounds that might induce this behaviour in its host.”

Finding out more could carry economic benefits because other Euderus species infect known agricultural pests. “It’s possible that Euderus parasitoids could be released in areas where they currently do not exist in order to help control particular pest insects,” says Weinersmith.